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SEMI C15-95 © SEMI 1995, 2002 2 5.2 Examples of Moisture Standar d s — Some examples of moisture standa rds an d the principles upon which they are based are as follows: • Permeation and effusion tube standards. These mu…

SEMI C15-95 © SEMI 1995, 2002 1

SEMI C15-95 (Reapproved 1102)

TEST METHOD FOR ppm AND ppb HUMIDITY STANDARDS

This test method was technically approved by the Global Gases Committee and is the direct responsibility of

the North American Gases Committee. Current edition approved by the North American Regional Standards

Committee on July 21, 2002. Initially available at www.semi.org October 2002; to be published November

2002. Originally published in 1995.

1 Purpose

1.1 This test will determine whether or not the quantity

of moisture delivered by a gaseous moisture standard is

in agreement with that predicted for the standard on the

basis of physical principles. It is expected that this

method will be used to validate moisture standards

before they are first placed in use, when deviation from

expected performance is likely or suspected, and at

other times as seems necessary.

1.2 The test method assumes the existence of a reliable

moisture analyzer. Although extensive precautions are

described to ensure the validity of the analyzer, it is

always possible that the test could yield a faulty result

through some unforeseen defect in the analytical

equipment.

1.3 Given that moisture generation is subject to many

pitfalls, it will be required that the predicted moisture

concentration delivered by the standard and the

measurement of that level by the analyzer be

independent, and that they agree. The level of

agreement can vary depending on the degree of

precision claimed for the moisture standard, but in any

case the measured and predicted results should be

within 10% over the entire range of the moisture

standard. If not, then the validation should be repeated

after verifying the physical measurement upon which

the prediction is based and any other parameters

deemed suspect.

2 Scope

2.1 This method is intended to be applicable to any

type of gaseous standard delivering a quantity of

moisture in the ppm range or lower, provided the

delivered moisture concentration is predictable on the

basis of fundamental principles of physics. Thus, any

standards which rely solely on characterization by

analysis are specifically excluded.

2.2 As part of this test, a procedure is described for

qualifying certain moisture analyzers considered

suitable for use in qualifying moisture standards.

Moisture analyzers intended for other applications may

need to satisfy different criteria.

2.3 As the most accurate and reliable moisture

analyzers currently available operate primarily in the

ppm range, this test focuses on validation of moisture

standards in this range. The need for ppb moisture

standards is addressed by specifying procedures for

reducing the output of ppm moisture standards to the

ppb range.

2.4 This standard does not purport to address safety

issues, if any, associated with its use. It is the

responsibility of the users of this standard to establish

appropriate safety health practices and determine the

applicability or regulatory limitations prior to use.

3 Limitations

3.1 If no higher level of accuracy is specified by the

user, then a moisture standard shown to be valid by this

method will generate moisture levels which deviate

from the expected values by no more than 20%. The

method leaves open to the user the option of using

sound statistical methods to specify that a given

moisture standard will generate levels showing a closer

agreement with expected values.

3.2 Validation based on this test method is good for a

limited time. The acceptable interval between

validations depends on the nature of the standard and is

not addressed by this method. Thus any citation of

validation results should be accompanied by the date(s)

on which validation was performed.

4 Referenced Documents

This method is based upon principles outlined in F.

Mermoud, M.D. Brandt, and J.J.F. McAndrew, “Low

Level Moisture Generation,” Analytical Chemistry, 63

(2): 198-202 (1991).

5 Device under Test (Test Specimen)

5.1 Definition of a Moisture Standard — A moisture

“standard” is defined to be any device capable of

delivering a flow of humidified gas at controlled

pressure, with no input being provided by the user

except nitrogen of UHP grade (moisture < 1 ppm) or air

of equivalent purity. Any device, such as a permeation

tube, which can provide a predictable amount of

moisture but does not include a means to deliver gas to

and from it, will not be considered a moisture standard

by itself. A device which requires gas of higher purity

than UHP grade will include a purifier capable of

purifying UHP nitrogen to this level.

SEMI C15-95 © SEMI 1995, 2002 2

5.2 Examples of Moisture Standards — Some

examples of moisture standards and the principles upon

which they are based are as follows:

• Permeation and effusion tube standards. These

must include purification means and some means

of regulating the outlet pressure. The moisture

delivery rate can be calculated based on the weight

loss of the tube.

• Cylinder standards. The “standard” will include a

regulator specified for use with the cylinder. The

moisture concentration delivered can be calculated

based on that added to the cylinder in preparation.

A minimum use pressure must be specified.

• Methods based on saturation of gas with water

vapor at a fixed temperature and pressure. The

moisture concentration in the gas may then be

calculated from a knowledge of the saturation

vapor pressure of water over a plane of the pure

phase of ice at the saturation temperature and of the

interaction virial coefficients of the gas-vapor

mixture. The two-pressure and two-temperature

methods are refinements of this approach requiring

additional chambers whose temperature and

pressure must be known.

5.3 Dilution of Standards — For all moisture

generation methods, lower concentrations can be

generated by dynamic dilution (i.e., by combining a

known flow of the standard gas with a known flow of

dilution gas). A diluted standard is acceptable

provided:

• The standard is validated at high concentration

(with the lowest concentration validated being no

more than 10,000 times the use concentration),

• The dilution gas has a moisture level below the

stated precision of the analysis (to be verified using

the same flow path as during the subsequent

analysis, and at the lowest flow rate to be used),

• The absolute accuracies of the dilution system

components are verified by comparison with a

reliable flow standard,

• The linearity of the dilution system can be

demonstrated over the entire range of operation.

The last criterion is particularly important whenever

some portion of the combined flow is discarded, as

mixing problems can easily arise at large dilution

factors.

6 Procedure

6.1 Validation will be performed using a hygrometer

whose output can be directly related to the moisture

concentration on the basis of physical laws or using an

analyzer which has recently been calibrated by

comparison with such an analyzer.

6.2 Hygrometer Qualification

6.2.1 Primary Hygrometers — Hygrometers can be

certified by the National Institute for Standards and

Technology (NIST) in the U.S., and similar certification

is available in other countries, although the lowest

concentration at which certification is available will

vary. Repeated certification at yearly or longer intervals

can be used to establish reliability. A hygrometer which

has been certified by a national standards laboratory to

1 ppm (or lower) moisture in air or nitrogen, and for

which reliability has been established over a period

greater than or equal to that which has elasped since

certification occurred, is acceptable for validation of a

moisture standard and will be referred to as a “primary”

hygrometer.

Because hygrometers which are suitable for use as

primary hygrometers may be less convenient or less

readily available than other hygrometers, the use of

other hygrometers for validation is also acceptable,

provided adequate precautions are taken to ensure that

they are operating correctly. These precautions (to be

described below) require using a moisture source to

compare a secondary hygrometer with the primary

hygrometer. The moisture source need not have been

validated in this case.

6.2.2 Validation of Secondary Hygrometers —

Secondary hygrometers may be validated by comparing

their output with a primary hygrometer when connected

in parallel to the same moisture source. The degree of

agreement required can vary depending upon the

precision to be claimed, but in any case the difference

in reading between the two hygrometers should be no

more than 10% at any point. Comparison should be

made at four moisture concentrations or at two per

decade of proposed operation, whichever is less. Half of

the comparisons should be made while increasing

concentration and half while decreasing, including at

least one comparison at the highest point of the

concentration range. The first and last comparisons

should be at the lowest point of the concentration range.

For example, in order to validate a hygrometer for

operation between 1 and 100 ppm, it would be

acceptable to generate moisture levels at 1, 30, 100, 50,

1 ppm (in that order) and compare the primary with the

secondary hygrometer. Comparison of the measured

level between the two 1 ppm tests provides a ready

indication of hysteresis in the system. If the secondary

hygrometer requires calibration using its own standard,

calibration should be effected before validation.

However, it is acceptable to adjust the calibration in

SEMI C15-95 © SEMI 1995, 2002 3

order to improve agreement with the primary

hygrometer.

6.2.3 Frequency of Hygrometer Validation — The

frequency of validation required for a secondary

hygrometer will vary dependent upon the performance

established for hygrometers of the same general type.

If three successive monthly validations show continuing

agreement (within 10%) with the primary hygrometer

without calibration adjustment, the validation interval

may be extended to three months. Similar extensions of

validation interval to six months and one year may be

made if no drift is apparent and no calibration

adjustments are necessary to bring the secondary within

10% of the primary at all points.

6.3 Validation Procedure for Moisture Standards

6.3.1 Validation Within the Range of an Available

Primary Hygrometer — If a primary hygrometer is

available, and its detection limit is no higher than the

lowest concentration at which it is desired to use the

moisture standard, this is the simplest case. Use the

standard to generate four concentration levels or two

per decade (whichever is less) in which the standard

will be used, half of the concentrations generated while

increasing concentration, half while decreasing,

including at least one comparison at the highest point of

the concentration range. The first and last comparisons

should be at the lowest point of the concentration range.

If the generated levels, calculated according to the basic

properties of the generator (e.g., weight loss of a

permeation tube) agree with the measured levels to

within 10%, the standard may be considered validated

with an accuracy of 10%. Higher validation accuracy

requires better agreement. It has been assumed that the

precision of the primary hygrometer, as determined by

a national standards laboratory, is much better than

10% and may be neglected for validation at the 10%

level. Validation at higher accuracy requires explicit

consideration of the certification precision.

If a standard only generates one level, connect it to the

hygrometer and allow to come to equilibrium.

Assuming the equilibrium level agrees with the

calculated level of the generator, disconnect the

hygrometer and bring it to equilibrium with either dry

nitrogen or ambient air. Then reconnect the moisture

standard and reestablish equilibrium between the

standard and the hygrometer. Disconnect again and

repeat a third time. If the three measurements agree

within 10% among themselves and with the generated

level, the standard is validated.

It is implicit in the above that the background moisture

level of the system be less than 50 ppb so that it can be

neglected for all measurements above 1 ppm with an

error no larger than 5%. We can usually neglect

background moisture in validating moisture standards

in this concentration range.

6.3.2 Validation Using Secondary Hygrometers — If

no primary hygrometer is available, then validation can

be performed using a secondary hygrometer. If the

lowest concentration to be generated using the standard

is no higher than the detection limit of the primary

hygrometer used for validation of the secondary

hygrometer, then the procedure is as above, except that

the secondary must first have been validated over the

entire concentration range which the standard will be

used to generate. (A secondary hygrometer and a

moisture standard can be validated simultaneously if

the primary and secondary hygrometers are connected

in parallel to the moisture standard.)

When assessing the accuracy of validation using a

secondary hygrometer, it is necessary to take into

account the precision of the comparison of the primary

hygrometer with the secondary hygrometer as well as

the precision of the comparison of the generator with

the secondary hygrometer.

7 Practice for Generating ppb Moisture

Standards

The detection limit of the primary hygrometer will

generally be on the order of 1 ppm, whereas it will

frequently be necessary to use the moisture standard at

concentrations below this level, by a factor of as much

as 10,000. Therefore, valid methods for reducing

concentration are necessary.

7.1 Dilution of Validated Moisture Standards — If a

moisture standard validated in accordance with Section

6.2 or Section 6.2.1 is diluted with a zero gas stream

satisfying the criteria of Section 5.3, then the resultant

diluted standard may be considered valid and may be

used for calibration of analyzers, such as APIMS.

7.2 Consideration of Background Moisture — In any

ppb or lower moisture measurement, it is necessary to

consider the background moisture level due to

outgassing in the sampling system and residual

moisture in the generator + analyzer. Often the

background will be sufficiently large that it must be

taken into account explicitly. The background moisture

level is the moisture concentration measured in the

absence of a moisture source (i.e., when the driest

carrier gas available is fed to the analyzer using the

same flow rate and flow path as is used for the sample

gas during analysis).

It should be noted that the importance of the

background level increases for measurements at low

concentrations, and that the background level is

sensitive to the materials of construction, to the design